The interest in deploying low-power nodes, such as pico base stations, home radio base stations (home eNodeBs), relays, remote radio heads and the like, in a network has constantly increased over the last few years. When a network employs low-power nodes in addition to regular base stations, the network is usually referred to as a heterogeneous network. The regular base stations are often referred to as a macro layer and the low-power nodes are often referred to as a pico layer. A purpose of employing low-power nodes is to enhance performance of the macro layer in terms of network coverage, capacity and service experience of individual users.
At the same time as interest for love-power nodes has grown, it has been realized that there is a need for enhanced interference management techniques to address the arising interference issues caused by, for example, a significant transmit power variation among different cells. Cell association techniques are also needed for more uniform networks.
In the Third Generation Partnership Project (3GPP), heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas. These areas may include so-called traffic hotspots, i.e. small geographical areas with higher user density and/or higher traffic intensity. Installation of low-power nodes, such as pico nodes, in these areas can enhance performance. Heterogeneous deployments may also be viewed as a way of densifying, or concentrating, networks to adopt for the traffic needs and the environment. However, heterogeneous deployments also bring challenges for which the network has to be prepared in order to ensure efficient network operation and superior user experience. Therefore, different interference management techniques, or interference management procedures, have been proposed.
One example of such interference management procedure relates to interference management for heterogeneous deployments. To ensure reliable and high-bitrate transmissions as well as robust control channel performance, maintaining a good signal quality is a must in wireless networks. The signal quality is determined by the received signal strength and its relation to the total interference and noise received by the receiver. A good network plan, e.g. good cell planning, is a prerequisite for successful network operation, but the cell planning is static. For more efficient radio resource utilization, the network plan has to be complemented by semi-static and dynamic radio resource management mechanisms. These mechanisms are intended to facilitate interference management and deploy more advanced antenna technologies and algorithms.
One way to handle interference is to utilize more advanced transceiver technologies, e.g. by implementing interference cancellation mechanisms in terminals, or user equipments (UEs). Another way, which can be complementary to the former, is to design efficient interference coordination algorithms, such as inter-cell interference coordination (ICIC) and transmission schemes in the network.
Inter-cell interference coordination (ICIC) methods for coordinating data transmissions between cells have been specified in Long Term Evolution (LTE) release 8. In LTE release 8, the exchange of ICIC information between cells in LTE is carried out via an X2 interface by means of the X2-Application Protocol (X2-AP protocol). The X2 interface and the X2-AP are known from 3GPP terminology. Based on this information, the network can dynamically coordinate data transmissions in different cells in the time-frequency domain and also by means of power control so that the negative impact of inter-cell interference is minimized. With such coordination, base stations may optimize their resource allocation by cells either autonomously or via another network node ensuring centralized or semi-centralized resource coordination in the network. With the current 3GPP specification, such coordination is typically transparent to user equipments.
Two examples of coordinating interference on data channels are illustrated in FIG. 1a and FIG. 1b. In FIGS. 1a-e, subcarriers, i.e. different frequencies, are vertically arranged along a vertical axis and time is expressed along the horizontal direction. Exemplifying data channels are denoted D1, D2, D3 and D4. In the example of FIG. 1a, data transmissions, such as D1, D2, D3 and D4, in two cells are separated in frequency, i.e. the no-data regions D1, D4 do not overlap in the vertical direction. The two cells, such as the pico and the macro, belong to different layers, i.e. macro and pico layers. By contrast, in the example of FIG. 1b, low-interference subframes are created at some time instances, such as at a center subframe of the three subframes shown for the macro, for data transmissions in pico cells by suppressing macro-cell transmissions in these time instances. This may, for example, enhance performance of user equipments which would otherwise experience strong interference from macro cells. For example, this applies to user equipments which are closely located to macro cells, or macro radio base stations. Such coordination mechanisms are possible already with the current specification.
Unlike for data transmission, current specifications limit ICIC possibilities for control channels. For instance, the mechanisms illustrated in FIG. 1a-b are not possible for control channels and are not possible for reference signals measured for mobility.
FIGS. 1c-e illustrate three approaches (1) (2), (3) of enhanced ICIC to handle the interference on control channels.
(1) as in FIG. 1c illustrates use of low-interference subframes in time.
In FIG. 1c, the vertical stripes indicate reduced interference to the control channels in the control region.
(2) as in FIG. 1d illustrates use of time shifts. It is noted that (2) has some limitations for Time Division Duplex (TDD) and is not possible with synchronous network deployments. Moreover, (2) is not efficient at high traffic loads. From the legacy terminal point of view, Cell-Specific Reference Signals (CRS) still need to be transmitted in all subframes, so there will still be inter-cell interference from CRS.
(3) as in FIG. 1e illustrates use of in-band control channel in combination with frequency re-use. In FIG. 1e, the grids indicate reduced interference to the enhanced control channels in the data region.
(1) and (3) require standardization changes whilst (2) is possible with the current standard.
Interference coordination techniques, as illustrated in FIGS. 1a-e, reduce the interference from a strong interferer, e.g. a macro cell, during other-cell, e.g. pico, transmissions. In doing so, the techniques assume that second cells, such as pico cells, are aware about the time-frequency resources with low-interference conditions and thus can prioritize scheduling of transmissions in those subframes for users which potentially may strongly suffer from the interference caused by the strong interferers.
As mentioned above, the possibilities to efficiently mitigate inter-cell interference to and from control channels are limited with the current standard. However, even less flexibility exists for dealing with interference to/from physical signals which typically have a pre-defined static resource allocation in the time-frequency space. In the following, some known techniques for interference cancellation are described.
In signal cancellation techniques, the channel is measured and used to restore the signal from a limited number of strongest interferers. This technique impacts receiver implementation and its complexity. Furthermore, in practice, channel estimation puts a limit on how much signal energy can be subtracted.
In symbol-level time shifting techniques, there is no impact on the standard, but it is not relevant for TDD networks or networks providing the MBMS service. This technique is applicable when a macro base station and a home base station are time-synchronized. This approach uses time shifting of transmission from the home base station relative to downlink frame timing of macro base station and uses power reduction, or muting, by the home base station and/or the macro base station, on the portion of a symbol(s) that overlap the control region of the macro or home base station.
A further technique completely mutes a signal in a subframe. In this technique, CRS are not transmitted at all in some subframes for energy efficiency reasons. This technique was proposed earlier in 3GPP. The technique is not backward compatible to Release 8 and/or 9 user equipments, which expect CRS to be transmitted at least on antenna part 0.
Given a very limited set of possibilities for interference cancellation listed above, there is a strong need for simple but yet efficient new techniques to resolve the CRS interference issue. A similar issue exists, for example, for synchronization and broadcast channels, where time shifts may be utilized to address the issue.
The need for enhanced ICIC techniques is particularly crucial when the cell assignment rule diverges from the Reference Signal Received Power (RSRP) based approach, e.g. towards a pathloss- or a ‘path gain’-based approach. This is sometimes also referred to as the cell range expansion when adopted for cells with a transmit power lower than neighbor cells. An idea of the cell range expansion is illustrated in FIG. 2, where the cell range expansion of a pico cell is implemented by means of a delta-parameter, aka bias or cell selection offset. The cell range expansion approach is also known as biased cell selection.
As discussed above, different interference coordination techniques, also referred to as enhanced ICIC (eICIC), have been discussed in the context of heterogeneous network deployments.
Now returning to the X2 interface, information to be signaled between radio base stations and from radio base stations, such as eNodeB, to a user equipment (UE) is described. It has been proposed that the signaling shall comprise the following:                one bitmap pattern to indicate an Almost Blank Subframe (ABS) pattern of Macro cell to Pico cell,        a second bitmap to indicate a subset of the subframes indicated by the first bitmap, which are recommended to the receiving node for configuration of restricted Radio Link Monitoring (RLM) and/or Radio Resource Management (RM) measurements, and        the pattern length and periodicity: Frequency Division Duplex (FDD)—40 ms: TDD—20 ms for downlink and/or uplink (DL/UL) configuration 1˜5, 70 ms for DL/UL configuration 0, 60 ms for DL/UL configuration 6.        
It is also proposed that the requested Radio Resource Control (RRC) signaling comprises RRC signaling for resource specific RLM/RRM measurements and Channel State Information (CSI) measurements, where the resources that can be used for measurements are indicated by patterns, such as an ABS pattern or a pattern for RRM/RLM. A definition of ABS subframes is described in the following. For an ABS subframe, user equipments may assume the following:                All ABS subframes carry CRS;        If Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), Physical Broadcast Channel (PBCH), System Information Block 1 (SIB1), Paging and/or Positioning Reference Signals (PRS) coincide with an ABS, they are transmitted in the ABS (with associated PDCCH when SIB1/Paging is transmitted);        Needed for legacy support;        CSI reference signals (CSI-RS) transmission on ABS is not determined yet;        No other signals are transmitted in ABSs;        If ABS coincides with Multicast Broadcast Single Frequency Network (MBSFN) subframe not carrying any signal in data region, CRS is not present in data region;        MBSFN subframe carrying signal in data region shall not be configured as ABS.        
According to the current state of the art, a measurement pattern per cell is to be decided. The measurement pattern is determined by a bitmap which is signaled to the user equipment, and it does not have to be the seine as the transmission pattern.
In a scenario, where load varies significantly among cells, it Is expected that different ABS patterns are configured in different cells. When determining the measurement pattern, the different ABS patterns Will need to be taken into account. Thus, only a small set of resources may be available for measurements. As a result, measurement quality may degrade and measurement time may increase.
Moreover, according to the current state of the art, a transmission pattern, or a ABS pattern, may be signaled to a neighbor radio node. The transmission pattern is determined by another bitmap.
With reference to the above mentioned scenario, the ABS pattern puts limitation on the network performance, in particular in terms what measurement pattern can be used.
In 3GPP DRAFT: R2-106449, to ALCATEL-LUCENT, titled “Signalling support for Almost Blank Subframe patterns”, made available on 9 Nov. 2010 (2010-11-09), XP050467151, RAN WG2, Jacksonville, USA, there is disclosed signaling for enhancing Inter-cell interference coordination. For example, X2 and RRC signaling is described. In one example, with a UE specific RRC signaling, a Pica eNB is able to indicate an appropriate ABS pattern to the UE based on its location (based on the strongest interferer). Alternatively, it is also possible to provide the UEs with two sets of ABS patterns (corresponding to a first Macro eNB and a second Macro eNB.
WO2009/129261 discloses systems and methodologies that facilitate resource management in a wireless communication system. a network cell in a wireless communication system (e.g., a macro cell) is configured to mitigate the effects of Interference on other surrounding network cells (e.g. femto cells embedded within the coverage of the macro cell). For example, a network cell can allocate control resources that overlap control resources of a nearby cell and assign resources within the region of overlap only to users that will not cause substantial interference to the nearby cell. As another example, a network cell can utilize a control channelization that partially coincides with a control and/or random access channelization of a nearby cell. The network cell can subsequently elect not to use the control resources in the coinciding region in order to enable the nearby cell to control the effects of interference though data scheduling.